1. Field of the Invention
The present invention relates to wireless receivers, and, in particular, to a method for despreading bit sequences that have been spread with Orthogonal Variable Spreading Factor (OVSF) codes.
2. Description of the Related Art
When a signal travels through a real-world environment, it often reaches a receiver by multiple paths. These paths occur as a result of the signal reflecting, diffracting, and scattering from various elements in the environment, e.g., mountains, trees, and buildings. Multi-path components are essentially time-delayed variants of a single signal. While, in some applications, these multiple components may result in interference, e.g., ghosting on the display of an analog-television receiver, Code Division Multiple Access (CDMA) systems intentionally make use of these multiple components.
The basic principle of CDMA systems is orthogonal coding, whereby, instead of assigning specific frequencies or time slots to each user of a system, the users are distinguished from one another by assigning codes. The codes fulfill the same role as frequency or time in frequency- or time-division systems, i.e., to keep the signals for different users from interfering with one another. In orthogonal spreading, a symbol is XOR-multiplied by a defined bit sequence called a code. If the code length is N bits, then each symbol is transformed to N so-called chips. The resulting chip rate, i.e., the number of chips per second (e.g., bits per second), is N times the original symbol rate (number of symbols per second). For example, the spreading code 1111 has a length, also called a spreading factor (SF) or Orthogonal Variable Spreading Factor (OVSF), of four. A single 1 will be spread to the sequence 0000 (1 XOR'ed with 1 gives 0), and a single 0 will be spread to the sequence 1111. In general, codes are not arbitrarily chosen, but rather, selected according to certain mathematical rules that provide sets of codes that are orthogonal to each other. Orthogonal codes have no correlation. Consequently, signals spread with codes that are orthogonal to each other do not interfere with one another. For a single connection, input data is spread with a particular code at the transmitter end. To recover the data, the same orthogonal code is used at the receiver end to despread the signal.
High-Speed Downlink Packet Access (HSDPA) is a standard for high-speed data transfer methodology employed in the Third-Generation Partnership Project (3GPP) to support the increasing use of mobile data services, including enhanced data transfer rates. In an HSDPA system, a High-Speed Physical Downlink Shared Channel (HS-PDSCH) is sent from a base station to every user in a given cell, in addition to other channels that may be available as part of the existing 3GPP standard. The HS-PDSCH channel is itself split into a variable number of (up to 15) subchannels (hereinafter, “HS-PDSCH channels” or simply “channels”), whereby users are differentiated both by scrambling codes and by time allocation of the channels in 2 ms timeslots called Transmission Time Intervals (TTI). Each user is allocated a code to access an HS-PDSCH channel during the appropriate TTI, and each user has access to an accompanying High-Speed Shared Control Channel (HS-SCCH), which carries control information used by the user's mobile device to decode the current data in the HS-PDSCH. While the actual number of HS-PDSCH channels is determined by information carried the in HS-SCCH, the present application will refer, generally, to scenarios employing all 15 HS-PDSCH channels.
Each of the 15 HS-PDSCH channels is spread using a different OVSF code with a spreading factor of 16. The receiver in a mobile device despreads all 15 channels within every 16-chip period. The despreading of an HS-PDSCH channel is essentially an accumulation process involving either addition or subtraction of received chip data based on the unique OVSF bit sequence of the channel. Received HS-PDSCH data is despread to a symbol by adding or subtracting the corresponding OVSF codes during each 16-chip period. The order of adding or subtracting for a given HD-PDSCH channel is dictated by the OVSF code corresponding to the channel's OVSF code number. To comply with the 3GPP standard, a conventional HSDPA receiver simultaneously despreads 15 HS-PDSCH channels using physical instantiations of both an independent OVSF generator and an accumulator for each channel. Each of the OVSF generators either produces the OVSF bits on-the-fly or uses a look-up table to read the OVSF bits. During each 16-chip period, the OVSF generator provides the appropriate OVSF bits to the corresponding accumulator, which despreads the chips into a symbol. For each instantiation, as many as 15 addition or subtraction operations may be performed to despread a single symbol, for a total of up to 225 (15×15) operations per symbol. Accordingly, independently despreading each HS-PDSCH channel requires much hardware and consumes much power. The same inefficiencies occur in OVSF despreading applications outside the HSDPA standard, such as performing Walsh, Hadamard, and other pattern transforms, including those used in ordinary (i.e., non-3GPP) CDMA OVSF despreading.